The present invention relates to a pressure control valve for vehicular hydraulic brake systems incorporating a valve arranged between an inlet and an outlet, having a control piston acting as a valve closure member, slidable against the force of a spring and cooperating with a sealing device encompassing the control piston, which carries on its part a valve seat and acts as a check valve from the outlet chamber to the inlet chamber.
In today's brake systems for automotive vehicles, pressure control valves are assigned preferably to the wheel brake cylinders of the rear axle and control the braking force at the rear wheels during a braking action dependent on the dynamic axle-load distribution.
A pressure control valve of the type referred to hereinabove is known from German Patent DE-AS No. 1,580,148. The pressure-reducing valve therein described includes basically a housing having an outlet port, an inlet port and a pressure chamber arranged therebetween. Guided in the pressure chamber and in further bores of the housing is a differential piston constructed as a stepped piston which is in its rest position pressed against a stop at the outlet port by the force of a spring. Pressure supplied to the inlet port propagates through the pressure chamber and past a sealing ring having a sealing lip inserted in annular grooves of the stepped piston to the outlet port, with the pressure prevailing at the outlet port exerting a force on the stepped piston acting in opposition to the spring force. Upon attainment of a specific pressure level, the stepped piston moves in an axial direction against the force of the compression spring and urges the sealing ring against a valve seat. A further pressure increase at the inlet will, however, assist the force of the compression spring so that a backward movement of the stepped piston is started. Now the pressure in the outlet is permitted to rise again up to a predetermined value which is sufficient to displace the stepped piston once more against the force of the compression spring and against the increased pressure in the pressure chamber. As a result, taking into account the spring force, the pressure built up in the pressure chamber will be transmitted to the outlet solely in ratio of the pressurized surfaces of the stepped piston.
The end of the stepped piston remote from the outlet is guided in a blind-end bore of the housing, in which pressure fluid will gather in a certain mounting position of the pressure-reducing valve which could escape through leaks. With the aid of an air cushion and another sealing ring, this pressure fluid is fed back into the brake circuit when the stepped piston moves upwardly. This mode of operation will, however, take place only if the pressure-reducing valve has assumed a specific mounting position in the vehicle. Moreover, the air cushion exerts an undefined force on the stepped piston, by which the change-over point of the pressure-reducing valve is displaced an appreciable amount. A further disadvantage of the arrangement described is that the change-over point is determined by the constructional dimensioning of the compression spring, without the possibility being afforded during the assembly to compensate for possible manufacturing tolerances of the compression spring. Even if auxiliary means were provided for a subsequent adjustment, this could only be effected by means of a complicated measuring operation by measuring the characteristic curve of the assembled valve. Also, a control of the change-over point can only be carried out by means of a pressure check. In addition, the comparatively complicated construction of the stepped piston causes high manufacturing costs. It is another essential disadvantage of the known device that the pressure in the wheel brake cylinders of the rear axle remains at a constant level when the brake is released until the pressure in the inlet chamber falls below the pressure level in the outlet chamber. In this state of operation, the braking effect of the rear axle is maintained, although the braking pressure in the front axle has already decreased considerably.